Add /topics and /drop-topic commands

[kimjune01]

Depends on #4940. Review after that PR merges.

Why two PRs

PR 1 (#4940) adds the backend — an opt-in union-find summarizer that maintains topic clusters. It changes nothing by default and can be evaluated on its own merits: quality-equivalent output, mandatory fallback to recursive, 57 tests.

This PR adds the user-facing commands that make the structured backend useful:

• /topics — see what the model remembers, with token counts
• /drop-topic N — remove one stale topic without wiping everything

Splitting lets the maintainer evaluate the data structure separately from the UX surface. The backend is the risky part; the commands are straightforward.

What it does

/drop-topic updates done_messages immediately (same pattern as /clear). Both commands refuse during background summarization. Without --chat-history-summarizer union-find, they show a guidance message.

Addresses #3607 (selective history control), #2219 (see/edit context), #948 (token breakdown with actions).

Test plan

• 67 tests passing, 565 total, zero regressions
• Integration test: feed multi-topic conversation → drop topic → verify done_messages excludes dropped content → verify forest rebuild

🤖 Generated with Claude Code and Codex

[kimjune01]

Demo output (mock model, simulated 40-message session)

Five distinct topics: database migrations, JWT auth, React components, CI/CD, payment processing.

aider> /topics

Chat history topics:

  1.    15 tokens - "I discussed The migration needs a down method too in case we need to rol"
  2.    15 tokens - "I discussed Can we also add an index on the verified column for faster q"
  3.    14 tokens - "I discussed Updated the token payload to include exp set to 24h from now"
  4.    10 tokens - "I discussed I will add a Skeleton component using react-loading-skeleton"
  + 337 tokens - 26 recent messages (not yet compressed)

Total: 434 tokens

aider> /drop-topic 1
Dropped topic 1 (15 tokens freed).

aider> /topics

Chat history topics:

  1.    15 tokens - "I discussed Can we also add an index on the verified column for faster q"
  2.    14 tokens - "I discussed Updated the token payload to include exp set to 24h from now"
  3.    10 tokens - "I discussed I will add a Skeleton component using react-loading-skeleton"
  + 337 tokens - 26 recent messages (not yet compressed)

Total: 419 tokens

To try it yourself:

cd ~/Documents/aider
git checkout feat/topics-codex-impl
source .venv/bin/activate
python -m aider --chat-history-summarizer union-find
# Have 30+ exchanges, then:
# /topics
# /drop-topic N

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